Washington: Discarded laptop batteries still have enough life in them to power slums in India and other developing countries, a new study by IBM India has found.

The research presented at a conference in San Jose, US, analysed a sample of discarded batteries and found 70 per cent had enough power to keep an LED light on for more than four hours a day for a year.

According to researchers at IBM India, many of the estimated 50 million lithium-ion laptop batteries discarded every year could provide electricity storage sufficient to light homes in developing countries.

It is possible to combine LED lights with solar panels and rechargeable batteries, however, using discarded batteries could make the approach far cheaper, according to MIT Technology Review.

"The most costly component in these systems is often the battery," said Vikas Chandan, a research scientist at the lab's Smarter Energy Group, who led the project.

"In this case, the most expensive part of your storage solution is coming from trash," said Chandan.

The IBM group, working with a hardware R&D firm called RadioStudio, opened the discarded laptop battery packaging and extracted individual storage units called cells.

They then tested those individually to pick out the good ones, and recombined them to form refurbished battery packs.

After adding charging dongles as well as circuitry to prevent overheating, they gave them to five users in Bangalore who lived in slums or operated sidewalk carts.